THE INFLUENCE OF CHEMICAL FACTORS 73 



high for at least an hour afterwards. The mechanism of these effects 

 is unknown, though it is probable that it stands in some relation to the 

 erythema of the skin produced. 



The Effects of Variations in the Oxygen Supply upon the 

 Respiratory Exchange. 



In 1873 Pfliiger expressed the view that the animal cell itself deter- 

 mines its own respiratory exchange, and that the oxidations are inde- 

 pendent within wide limits of the oxygen supplied to it. This view 

 was based Almost exclusively on speculative considerations of a teleo- 

 logical character and on the assumption that the oxygen supplied to the 

 cells is normally in excess of their requirements. When later the 

 point of view of chemical dynamics was applied by Thunberg [1905] 

 to the problem of oxidations taking place within the living body, he 

 thought it obvious that variations in the oxygen supply must cause 

 variations in the intensity of oxidations. 



According to Thunberg we have a reaction between oxygen and 

 certain substances. The velocity of the reaction (consumption of 

 oxygen per unit time) depends upon the number of molecules able to 

 take part in it, and if the molecules of one of the substances (the 

 oxygen^ become fewer in number the velocity of the reaction must 

 decline, unless indeed their number is so large that it can be considered 

 as infinite compared with the number of molecules with which it has 

 to react. According to this view, which is fundamentally opposed to 

 that of Pfliiger, the oxidative processes must increase with increased 

 oxygen pressure and gradually approach a maximum as shown in 

 fig. 21. 



Thunberg, like Pfliiger, assumed that the oxygen supply to the 

 tissues in the cases investigated by him was large enough to maintain 

 a positive oxygen tension within the cells. 



Thunberg looked upon the catabolism of the nutritive material 

 as a comparatively simple process and above all an oxidation. It 

 must be remembered, however, that according to modern views the 

 breakdown of any substance takes place through a number of definite 

 intermediate stages. The oxygen must take part in the reaction at 

 one of these stages and may conceivably take part in it at more. At 

 each of these stages the reaction proceeds at a certain rate, depending 

 no doubt on the number of molecules taking part in it, but also on the 

 " specific velocity " of the reaction in question, and the reaction velocity 

 of the whole process will be determined at least approximately by the 



